Ad hoc networks are self-forming networks including a number of nodes which can operate with or without any fixed infrastructure, and in some cases the ad hoc network is formed entirely of mobile nodes. An ad hoc network typically includes a number of geographically-distributed, potentially mobile nodes which are wirelessly connected to each other by one or more logical links (e.g., radio frequency communication channels). The nodes can be fixed or mobile and can communicate with each other over a wireless media with or without the support of an infrastructure-based or wired network. Logical links between these nodes can change dynamically in an arbitrary manner as existing nodes move within the ad hoc network, as new nodes join or enter the ad hoc network, or as existing nodes leave or exit the ad hoc network. A single-hop logical link can only exist between two nodes when they are within direct communication range. A multi-hop logical link can only exist between two nodes whenever a set of single-hop logical links can be used to construct a path between the nodes. Such multi-hop logical links are either instantaneously coherent (e.g. all single-hop links are present at the same time) or deferred coherent (e.g. all single-hop links are expected to be present or were present over a period of time).
Nodes can operate in at least two different modes within a network: an autonomous ad hoc mode and an infrastructure mode. In infrastructure mode, typically only one communication hop is used from a mobile node to a base station (i.e. a special node with an infrastructure connection). An autonomous ad hoc network operates in the absence or presence of infrastructure components such as a base station or Wireless Local Area Network (WLAN) access point (AP). In autonomous ad hoc mode, communications single-hop or multi-hop over logical links locally between nodes. Such nodes are sometimes referred to as peer nodes or peers in this context.
A node typically includes an ad hoc interface such as an IEEE 802.11 interface. IEEE 802.11 communication systems allow for “proximity-based” communications. For example, when two nodes are moving about, for example, in an urban area, those nodes can communicate within a range of approximately 50 meters of each other. IEEE 802.11 protocols which allow sleep cycling of the mobile nodes of the population assume that a particular mobile node can rely on the presence of other nodes or access points (APs) in close proximity at any given time. For instance, IEEE 802.11-based protocols assume that the availability of internet access points (APs) which provide nodes in the network with access to different services on the Internet. The APs provide signaling to other nodes which are traveling about the network in a sleep mode and periodically wake them up as needed. This approach is similar to approaches taken in cellular-based networks which rely on a central base station to provide this type of scheduling and discovery functionality. As such, a typical IEEE 802.11-based ad hoc interface continuously scans for other nodes in its ad hoc network, and the IEEE 802.11 interface is constantly running. Operating a node in infrastructured mode produces relatively longer battery life than autonomous ad hoc mode since the node can implement a power-save mechanism such as that specified in the IEEE 802.11b standard. Operating a mobile node in ad hoc mode can reduce the battery lifetime by a significant degree (e.g., factor of 10) when compared to infrastructured mode.
In an autonomous ad hoc network including a number of nodes, processes sometimes referred to as service discovery or peer discovery can be used so that a particular node can recognize when it encounters another node in its proximity which subscribes to the same service or a service the node is interested in, and can quickly alert the user that it is approaching another node in its area.
However, in many autonomous ad hoc networks, the density of nodes in the population is relatively low and contact between nodes is sporadic since nodes are spread out and only occasionally come within proximity of other nodes in the particular ad hoc network (e.g., a user walks around a metropolitan area with his node during the day; however, the user only sporadically runs into other nodes that they can communicate with in an autonomous ad hoc mode). As such, nodes randomly or semi-randomly come into contact with each other at different times.
In such networks, it is undesirable that these nodes have their ad hoc interface constantly running since much of the time the ad hoc interface will not be utilized and is therefore consuming power with no benefit to the user of that node. In other words, continuous scanning for other nodes can unnecessarily consume current and deplete battery power quite quickly.
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